1. Field of the Invention
The object of the present invention is an integrated circuit protected against ultraviolet rays. It can be applied to integrated circuits made by MOS technology and, especially, to integrated circuits that include floating-gate memory cells.
It is common practice to use floating-gate memory cells to customize an integrated circuit when it comes off the production line and after the testing sequence. This customizing is designed, inter alia, to establish the various access paths and to permit or bar the reading of memory zones. These memory cells are then used as fuses. They are, of course, also used in a standard way in integrated circuit memory zones.
These floating gate memory cells are sensitive to ultraviolet rays which undo the storage of the charges at the floating gate. Now any user can subject a circuit to the ultraviolet rays. It is therefore important, in certain applications, to protect the circuit against the ultraviolet rays, for example in applications requiring high security with respect to the keeping of information in a memory. Thus, it is necessary to ensure that certain users can have total immunity to ultraviolet rays for a number of years.
2. Description of the Prior Art
In a known way a metal mask, made of aluminium for example, is placed on the integrated circuit. Thus, for example, UUPROMs (Ultraviolet Unerasable PRogrammable Memories) are made out of EPROM cells. Experience shows that the protection provided by these cells against ultraviolet rays is limited in time.
In a first exemplary embodiment, this protection lasts only a few weeks. This is not enough for certain applications.
In this example, a metal mask is placed on the cell in a horizontal plane. This metal mask isolates the element of the cell to be protected, namely the floating gate, from the ultraviolet rays. Since, however, there is no shielding between the plane of the substrate and that of the metal mask, ultraviolet rays may reach the floating gate indirectly, by reflection between the substrate and the metal plane, thus undoing the storage of the charges that have collected therein: the immunity to ultraviolet rays does not go beyond a few weeks.
To circumvent this lack of efficiency, a second exemplary embodiment uses metal projecting features or projections vertical to the metal plane. These projections are anchored in the substrate around the cell to be protected. In fact, these metal projections are anchored in diffusions that are of the same type as but are more highly doped with impurities than that of the substrate. They are contact-making diffusions, for example P.sup.+ diffusions made in a P type substrate.
A floating-gate memory cell should, however, have its source, its drain and its control gate connected to conductive lines conveying the control signals that enable the reading, programming or erasure of this cell. Even if this cell is isolated with the metal projections anchored in P.sup.+ contact-making diffusion, it must be possible, all the same, to make these connections to the conductive lines. This can be done for the source and the drain which in the present example of a P type substrate, are made by N.sup.+ type diffusions. It suffices to make a N type well within which the contact-making P.sup.+ type diffusion will be made to anchor the metal projections Rm. And two N.sup.+ type diffusions will be made straddling the substrate and the well. One of them, for example, will be the source of the cell while the other, for example, will provide for the metal contact of a conductive line. It is in fact necessary to make an N type well such as this for it is technologically not possible to make a P.sup.+ type diffusion directly in an N.sup.+ type diffusion, because of their respective levels of impurity.
It is the connection of the control gate, which is formed by a polysilicon region, that raises problems. For, the polysilicon region which constitutes the control gate is above the plane of the substrate but beneath the metal plate. It prevents a vertical projection, as described, at its position. It is possible neither to cut the polysilicon gate nor to connect the control gate to the metal plate which is connected to a potential: the control gate would be short-circuited. In practice, there are no metal projections at the connection between the control gate and the exterior. The cell is not entirely isolated.
It is true that, in this example, the possible inlets for ultraviolet rays have been reduced, but these rays can still get through, via the polysilicon control gate. And, by reflection between the metal plane and the substrate, these rays could reach the floating gate. The period of immunity is thus increased by a few months. For applications requiring a minimum immunity of four to five years, the result is unsatisfactory.
In a third example of an embodiment, the trace of the polysilicon control gate was then modified so as to attenuate the rate of reflection of the ultraviolet rays. An example of an embodiment such as this is shown in FIG. 1. This top view of the circuit shows two metal lines representing the impact on the substrate of the vertical projections of metal. These lines are depicted, in FIG. 1, by a zone demarcated by thick lines.
FIG. 1 also shows a polysilicon line, cross hatched in the drawing, which goes through the two metal lines without being in contact with them. On the inner side of the protected zone, the polysilicon line makes several 90.degree. turns, along a directing axis. The two metal lines follow the winding path of the polysilicon line on either side of it. By these intertwined metal and polysilicon zigzags, the reflections of the ultraviolet rays are highly attenuated and several years of immunity to ultraviolet rays are thus obtained.
However, this is achieved to the detriment of the area occupied by the cell. For, these zigzags along an axis of direction entail a high cost in terms of space. And their cost in terms of space is all the higher as the desired period of immunity is great. Thus, in the current state of the art, four years' immunity is obtained for a length of the order of 200 .mu.m on the axis of direction of the zigzags. Given that, in this same state of the art, the EPROM cell normally occupies an areas of the order of 180 .mu.m.sup.2, it is seen that this increase in area is a major drawback in practice, for it is always sought to reduce the areas with a view to ever greater integration density. Moreover, this addition of materials leads to additional manufacturing costs.